Factor completely. $4x^2-28x+40=$
First, let's factor out the greatest common factor. If the leading coefficient is negative, we'll also factor out $-1$. The result will be something like this, where the blanks are coefficients: $_{\llcorner\!\lrcorner}\!(_{\llcorner\!\lrcorner}\! x^2+\, _{\llcorner\!\lrcorner}\! x+\, _{\llcorner\!\lrcorner}\!)$ Then, we can try to factor the sum like this: $_{\llcorner\!\lrcorner}\!(_{\llcorner\!\lrcorner}\! x+\, _{\llcorner\!\lrcorner}\!) (_{\llcorner\!\lrcorner}\! x+\, _{\llcorner\!\lrcorner}\!)$ Factor out a the greatest common factor The greatest common factor is $4$. $\begin{aligned} &\phantom{=}4x^2-28x+40 \\\\ &=4(x^2-7x+10) \end{aligned}$ [How did we find the greatest common factor?] Factor $x^2-7x+10$ $\begin{aligned} &\phantom{=}4x^2-28x+40 \\\\ &=4(x^2-7x+10) \\\\ &=4(x-2)(x-5) \end{aligned}$ [I want to see this step in more detail.] Answer $\begin{aligned} &\phantom{=}4x^2-28x+40 \\\\ &=4(x-2)(x-5) \end{aligned}$